To nominalise means to form a noun from a verb or adjective. That’s according to the dictionary – the OED, since you’re asking – put out becomes output, and true becomes truth. We can use the word ‘nominalisation’ more broadly to refer to the way writers turn events, states or ideas into noun phrases, thus making it easier to talk about them. Let’s take an example – you’re arguing with your sister about the remote control again (you two should really sort that out) and you say to her:

You are being really selfishness and unreasonable. You spend hours watching soaps which are really boring and which they are not artistic or worthwhile.

She replies:

My purported selfishness and lack of reason are not the issue, dear sibling, but your inability to appreciate the finer subtleties of soaps.